chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 47 



families) would have a limited range ; but, as is to some extent 

 the case 'in tropical America east of the Andes, every such 

 group would be represented over the whole area, bjr countless 

 minute modifications of form adapted to local conditions. 



2. One great physical barrier would, however, even then 

 exist ; the hot equatorial zone would divide the faunas and floras 

 of the colder regions of the northern and southern hemispheres 

 from any chance of intermixture. This one barrier would be 

 more effectual than it is now, since there would be no lofty 

 mountain ranges to serve as a bridge for the partial interchange 

 of northern and southern forms. 



3. If such a condition of the earth as here supposed con- 

 tinued for very long periods, we may conceive that the action 

 and reaction of the various organisms on each other, combined 

 with the influence of very slowly changing physical conditions, 

 would result in an almost perfect organic balance, which would 

 be manifested by a great stability in the average numbers, the 

 local jrange, and the peculiar characteristics of every species. 



4. Under such a condition of things it is not improbable that 

 the total number of clearly differentiated specific forms might be 

 much greater than it is now, though the number of generic and 

 family types might perhaps be less ; for dominant species would 

 have had ample time to spread into every locality where they 

 could exist, and would then become everywhere modified into 

 forms best suited to the permanent local conditions. 



5. Now let us consider what would be the probable effect of 

 the introduction of a barrier, cutting off a portion of this homo- 

 geneous and well-balanced world. Suppose, for instance, that a 

 subsidence took place, cutting off by a wide arm of the sea a 

 large and tolerably varied island. The first and most obvious 

 result would be that the individuals of a number of species 

 would be divided into two. portions, while others, the limits ot 

 whose range agreed approximately with the line of subsidence, 

 would exist in unimpaired numbers on the new island or on the 

 main land. But the species whose numbers were diminished 

 and whose original area was also absolutely diminished by the 

 portion now under the sea, would not be able to hold their 



